Physics

World Year of Physics

What is Physics?
Physics is the study of the fundamental features of the universe. These features apply to all of the stars, the Earth, and everything upon the Earth, including the billions of atoms in your body. Specifically, physics studies the following:

• The dimensions of space and time (and others)
• Matter and energy
• The fundamental forces
• Waves and quantum mechanics

What has Physics Done for Me?
Physics, through pure science research, aims to explain all of the fundamental interactions of matter and energy. This knowledge can then be applied: physics has facilitated many of the greatest technological advancements in the world. For example:

• Every device that uses electricity, including every electronic device - that is a huge list!
• Scanners used in the health service to see inside your body (X-rays, ultrasound, PET, MRI)
• Transport technology (cars, buses, lorries, trains, planes, ships)
• Space technology (rockets, spacecraft, satellites, telecommunications, weather forecasts, GPS)

The list is extensive. Almost every technology that you take for granted today, at some level, relies on the knowledge of physics. Even other pure sciences such as chemistry, and hence biology, rely to some extent on physics (e.g. laboratory tools).

Physics: Dimensions

The classic dimensions of physics are the three dimensions of space and one of time. If we also add mass then we can express the "dimensional" composition of various well known physical and mechanical attributes. The dimensions are expressed as follows: M : Mass : kilogram : kg L : Length : metre : m T : Time : second : s

Attribute	Example
Area: L2	The area of a rectangle of sides length 3cm and 2cm is 3 x 2 = 6 cm2
Volume: L3	The volume of a box of sides 3cm, 2cm and 10cm is 3 x 2 x 10 = 60 cm3
Density: ML-3	The density of water is approximately 1 gram per cubic centimetre (1 g cm-3).
Velocity: LT-1	The speed of light is approximately 300,000,000 metres per second, or 186,000 miles per second! Technically, velocity includes the direction of travel too (i.e. it is a vector).
Momentum: MLT-1	A 2 kg mass travelling at 5 metres per second has a momentum of 10. Note: Total momentum is always conserved. So for example, if the 2 kg mass exploded the total momentum would still be 10, even though bits may be travelling faster and in different directions.
Acceleration: LT-2	The rate at which the velocity of an object changes (acceleration) is proportional to the force acting on it. If you have heard of g-force then one g is 9.81 metres per second per second. In other words if you accelerate at one g, then every second your velocity changes by 9.81 metres per second. This is the rate of free fall, the rate you accelerate towards the Earth when you jump out of a plane. However, as you fall air resistance opposes the force of gravity until eventually you reach a terminal velocity and further acceleration stops. (But by then you may be doing over 100 miles per hour, unless you have a parachute!)
Force : MLT-2	The external force acting on an object that is accelerating is equal to the acceleration multiplied by the mass. So a 2 kg mass accelerating at 10 ms-2 has a net external force acting on it of 20 Newtons [named after Sir Isaac].
Energy : ML2T-2	Energy is the amount of "work" done when a force moves an object through a given distance. So a 20 N force pushing an object 5 metres has done 100 Joules of work (or energy). The energy of a moving object, its kinetic energy, is calculated using the formula E = 1/2 mv2. When mass is converted into energy in a nuclear reaction the formula is E = mc2, the well known equation of Albert Einstein, where c is the speed of light. For example, if just one gram of mass is converted into energy the amount of energy released is 0.001 x 300,000,000 x 300,000,000 = 90,000,000,000,000 Joules. That is equivalent to the heat from one million electric fire bars (of 1 kW each) being left on for 90,000 seconds (25 hours) - that's hot stuff!
Power : ML2T-3	Power is the rate at which work is done, or the rate at which energy is converted from one form into another [note that energy is always conserved: it cannot be created or destroyed]. So, back to the above example, if the object was pushed over that distance in 2 seconds then the power was 100 / 2 = 50 Joules per second (or 50 Watts).

Physics: Forces

Strong and Weak forces act over very short distances in the nucleus of atoms. They stop the positively charged protons in the nucleus from repelling each other, and hold together the building blocks of the subatomic particles found in the nucleus. (Neutrons and protons consist of quarks.)

The Electromagnetic force is widely experienced in everyday life. When an electrical charge is accelerated it emits energy in the form of electromagnetic waves. The spectrum of these electromagnetic waves range from radio waves, to TV broadcasts, microwaves, infrared, the colours of visible light, ultraviolet, X-rays and very energetic gamma rays. An electromagnetic wave consists of two fields, or forces: the electric field and at right angles to it the magnetic field.

Electric charges are positive or negative. Like electric charges repel, and opposites attract.

Magnets are a dipole consisting of a north and south pole. Like magnetic poles repel and opposites attract. For example, the tip of a compass needle that points to the North Pole is the south pole of the magnet in the needle. A magnetic field is closely related to an electric field: a magnetic field can be produced from an electric current. If an electric current, or charge, travels in a loop then a magnetic field will flow around that loop.

Gravity is the force that pulls you towards the Earth. All mass generates a gravitational force and all masses attract each other. (Though relatively recently an effect named "dark energy" has been observed which is apparently causing matter in the universe to repel, resulting in an accelerating expansion of the universe.)

Some physicists suspect that all of the above forces may be different attributes of one unified force.

Physics: Waves

Waves oscillate at a particular frequency, that is the amplitude of the wave changes through a complete cycle so many times a second. The wavelength is the distance between two consecutive peaks, or troughs, in the wave. All of the following are examples of waves: the ripples on the surface of water, a colour of light, and sound. Even earthquakes send shock waves through the Earth and around its crust.

In transverse waves the oscillations occurs in a plane perpendicular to the direction the wave is travelling (or propagating), e.g. on the sea the wave oscillates up and down, whilst the wave front propagates horizontally. In longitudinal waves the oscillations occur along the direction of propagation. Electromagnetic waves, including light, are transverse waves. Sound is a longitudinal wave.

If waves collide with each other they are said to interfere. Interference results in a smaller or larger amplitude at the point where the waves collide. The resultant amplitude is the sum of the amplitudes of the colliding waves, at that point. For example, if two similar waves collide but one is at the top of its amplitude (e.g. 2) and the other is at the bottom of its amplitude (-2) then at that point the resultant amplitude will be zero. However, if two similar waves are in phase where they collide then the amplitude at that point will be larger (e.g. 4). Interference accounts for a lot of interesting effects from waves. For example, if a plane (straight) wave is blocked by a barrier which has two holes in it, then the waves emerging from the two holes on the other side interfere to produce an interference pattern. (Also, the waves emerging from the holes have a diffraction pattern.)

Physics: Relativity

In relativity things start to get weird! However, these theories are well established and the effects are proven!

Special Relativity focuses on the relativistic effects that take place when objects are travelling near, or at, the speed of light relative to other objects. The faster the relative speed the greater the effect, and as the speed tends towards the speed of light the effect tends towards infinity. So what are the effects? Well, for objects moving relative to you, you observe that:

• Distances of moving objects are smaller than when stationary!
• Time runs slower on moving objects than when stationary!
• Mass increases for moving objects!

Yes it is hard to believe, but it is a proven fact. It does not just apply to people, it is a physical phenomena that applies to every object in the universe.

General Relativity focuses on the effects that gravity has on the dimensions of space and time. As we have just seen above, space and time can change, and the gravitational field produced by a mass bends space and time around it. So a beam of light passing nearby a large mass, such as our Sun, will bend slightly. This effect has been experimentally verified. Also, a gravitational field slows down time. So if you are close to a massive object (e.g. the Earth) time runs slightly slower than if you were in space. This effect, along with the time effect of special relativity, have been verified by taking a very precise atomic clock in an aeroplane. When the mass of an object gets really large, its gravitational field can be that strong that even light cannot escape it. This is a Black Hole. Near the edge of a black hole time slows right down. If you looked back at the Earth, for example, in seven days of your time frame you may see five billion years pass on Earth!

Physics: Quantum Mechanics

All radiated electromagnetic energy, including light, is in the form of waves, but it also behaves like a particle too. An individual packet of electromagnetic energy is called a photon. All particles, especially atomic sized particles, behave as waves too.

Quantum Mechanics is concerned with representing the behaviour of photons and (small) particles. The theory suggested some pretty bizarre behaviour, which has been proven by experimentation.

Quantum mechanics is about waves and probability, and the fact that small systems (such as the atom) force particles to occupy discrete energy levels: a finite, quantum, energy difference exists between neighbouring energy levels.

The wave associated with a particle is a probability wave, and every particle has such a wave. The wavelength of the particle is inversely proportional to its momentum. So a slow moving particle would have a relatively long wavelength, but if it moves faster its wavelength gets shorter.

If a particle is confined to a finite space, for example a box, then the amplitude of the probability wave is [just about] zero outside the box, because you would not expect to find the particle outside the box, would you? Similarly, inside the box right at the edge the amplitude is zero. That means that all waves inside the box must fit completely in the box: the parts of the waves touching the edge must always be at zero amplitude. That means that only certain wavelengths fit properly inside the box, depending on the size of the box. Hence, only specific values of momentum (and energy) can exist inside the box. This gives rise to the idea of the quantum energy states mentioned earlier. The box example applies to atoms too: in this case electrons are confined to the space of the atom (but it is not box shaped).

Certainly Not?
Then along came this guy called Heisenberg. He said that if you wanted to know where a particle is (such as an electron in an atom) you would have to do an experiment to observe its position. For example, you could fire a photon at the electron and see how the photon was deflected or reflected back. However, a photon has momentum of its own and behaves like a particle, so when it hits the electron it "knocks" the electron and changes its momentum and position. So what this means is that the act of observing the particle (the electron in this case) has changed the state of the particle. This led to the theory of the uncertainty principle. The uncertainty principle states that you can never know, with 100 percent accuracy, the momentum and position of a particle simultaneously. If the momentum is known with a high degree of accuracy, or certainty, then the position is known with less accuracy; and vice a versa. Similarly for energy and time. But it then turned out that even the particles themselves had this fundamental haze of uncertainty associated with them, even without us humans doing experimental observations. In other words, the uncertainty principle always applied, to all particles and photons.

Now things even get more bizarre! Going back to our particle in a box, it turns out that:

1. Unless something interacts with the particle, it can occupy multiple states simultaneously - contrary to what common sense tells you (it is like saying that the speed of the particle is 2, 4, 8, and 16 miles per hour, simultaneously!)
2. There is actually a finite probability that the particle can be outside the box at any given instant! Yep, even a box with six solid, and apparently impenetrable, walls!

It was the bizarre probability stuff that prompted Einstein to famously say, in disbelief, words to the effect of God does not play dice. You too may be thinking that it is just a theory, and the theory is wrong. You are not the only one!

Strange but True
Experiments have demonstrated these bizarre effects. The effect in point 1 is now being used in laboratories to build simple prototypes that may lead to a new generation of super computers: Quantum Computers. In traditional computers a bit in memory contains either the value 1 or 0. The new quantum computers are using qubits which can simultaneously represent the value 1 and 0. This presents the opportunity to perform some types of calculations at much faster speeds than traditional super computers. For example, it has been said that traditional encryption algorithms, which would take years to crack on traditional computers, may be crackable in one second or less with an equivalent quantum computer.

Oh and regarding point 2, that effect has been in use in electronics for many years, e.g. the tunnel diode.

The consequences of quantum mechanics have significant implications for the fundamental features of the universe. It also shows that you cannot always rely on "common sense".

Physics References

BBC Science

Google Physics

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